In sheets processed and printed in a predetermined way, a sheet having a tape stuck thereon to mend a breakage or having a partially lost part by tearing is not suitable for distribution and thus should be separated. Accordingly, an inspection device and a sort counter for the sheets require a mechanism and a method for separating unsuitable sheets by detecting and judging which sheet has a foreign material such as a tape or a lost part. In order to solve this problem, there is a method for identifying a foreign material adhering to a sheet by irradiating ultrasonic waves to the sheet in order to measure the attenuation of the ultrasonic waves passing through the sheet.
However, detection sensitivity of the conventional detection method is not enough to correctly detect a small foreign material or a small lost part.